[AlexanderHowl]

Pulsar planets are most likely captures (they are too close to have formed at their current radius, and the loss of mass during a supernova would have had any surviving planetary cores migrate outwards rather than inwards). The few planets we have found around white dwarfs are likely captures as well.

With the age of the Milky Way being ~13 billion years, there was plenty of time for supernova cascades (chains of supernovas that lead to star formation, which leads to supernovas, whuch lead to more star formation) to occur during the first billion years. 100+ solar mass stars burn through their fuel quickly and explode in less than 100,000 years, creating the shock waves that would lead to further stellar evolution. Even if you had 10 million years between supernova cascades, the Milky Way could have had dozens of supernova cascades before everything calmed down (10,000s of supernova per year during the supernova cascades).

After the first billion years, there would have been more than enough deuterium and helium-3 having been consumed to slow down the process. Metals would have formed planets, either rogues forming in interstellar space or bound planets forming around smaller stars. As the first white dwarfs formed around two billion years after the end of the supernova cascades, they would have captured rogue planets or stolen bound planets. Some of them would be 10 billion years old now.